There are generally two known techniques conventionally used to dry etch wafers: a batch wafer reactor or a single wafer reactor. In batch wafer reactors, a plurality of wafers are placed on powered substrates. Since a plurality of wafers are etched simultaneously, speed is not critical, and therefore batch reactors may operate efficiently at a low power density and at a process pressure of 0.001 to 0.5 torr in the Reactive Ion Etch (RIE) regime. The minimal heat generated by the etch process can be efficiently dissipated from the wafer to the substrate due to the low power density regime batch systems work within.
On the other hand, single wafer reactors such as, for example, plasma mode or magnetron ion etchers (MIE), can utilize high power densities to achieve high etch rates. Operating the single wafer etcher under a low pressure (5-500 millitorr) has also been found to aid anisotropic etching by reducing the concentration of species which may deflect the directional etching ions. Unfortunately, low pressure slows down the etch rate by limiting the concentration of available etchants.
In order to be competitive with a batch etcher, it is necessary for a single wafer etcher to etch at a faster rate than the batch etcher. The most frequently used method for increasing the etch rate is to increase the power density to the plasma in contact with the wafer. This results in a much higher heat load applied to the wafer that must be dissipated to prevent wafer thermal runaway which may promote unwanted lateral (isotropic) etching. Additionally, the very low pressures used to help control the anisotropic etching severely reduces (and practically eliminates) dissipation of heat through the surrounding atmosphere. Thus, it has been necessary to develop techniques to improve the cooling of a wafer in a single wafer reactor operating at high power densities and at low pressures.
One common cooling technique involves exposing the back side of the wafer (opposite the etch surface) to helium gas. A device known as a helium chuck has been developed specifically for cooling a wafer. A top layer of the chuck, which is proximate the silicon wafer to be etched, is cooled by a water system and is enhanced by a thermally conductive gas to transfer heat from the hot wafer to the cooled top layer of the etcher. Helium gas has typically been selected for use in the chuck because helium has the highest thermal conductivity of any known gas. However, other thermally conductive gases, such as, for example, neon or argon, may be used. A chuck requires input and output lines through the powered cathode to allow the flow of helium gas. Helium gas passes through the conductive layer, transfers heat from the back of the wafer and is then expelled.
The top layer of the helium chuck is powered in MIE and RIE (as well as some other plasma mode etchers) which may be used. In the RIE mode a high D.C. self-bias voltage develops within the chuck along with RF potentials resulting in significant D.C. and A.C. potential drops between the powered top layer and the grounded components coupled to the top layer, including the helium chuck cooling gas input and output lines. For example, in an RIE reactor, RF peak-to-peak voltages are around 3000 volts and D.C. voltages are commonly in excess of 1000 volts. Furthermore, during plasma ignition, the RF voltages may be considerably higher than 3000 volts. These high voltages result in a large potential drop within both the helium input and output lines. The potential drop frequently causes an ionization avalanche of helium which results in arcing, severe damage to the hardware and degradation of the wafer etching.
An ionization avalanche is caused by free electrons that have gained sufficient energy to ionize helium atoms. When other electrons are stripped from atoms, ions are created and more electrons are freed to create more ions. When enough energy is present, this process continues to progress creating an "avalanche" effect which eventually results in an electric arc. In a helium chuck there is more than enough energy available in the potential drop to create an avalanche.
The use of increased power combined with lower pressure in a single wafer etcher has created the need for a wafer cooling device. The addition of a wafer cooling device such as a helium chuck has added the problem of an ionization avalanche resulting from the potential drop across the helium chuck. Thus, there is a need for a way to prevent or greatly reduce ionization avalanches in a helium chuck cooling device.